1. Field of the Invention
The invention relates to a catalytically active material, more particularly a material which acts as an oxidation catalyst, and to a method for producing such a material, and to its use. The invention further relates to an article equipped with a catalytic highly heat-resistant coating on at least one face thereof, to improve self-cleaning.
2. Description of Related Art
Catalytic materials have been known in chemistry for many years. Special recognition have the systems employed in heterogeneous catalysis, for example in post-combustion of exhaust gases. Such systems are usually configured so that the catalytically active material is applied on the surface of an inert ceramic carrier of high porosity. Typically in this case the active catalytic materials comprise precious metals, preferably platinum, making such a component which is colloquially referred to as a “catalyst” correspondingly expensive, depending on the content of catalytically active material.
Therefore, in order to avoid high material costs, alternative materials have repeatedly been tested for their suitability for oxidative heterogeneous catalysis. Mixed oxide catalysts have become of particular importance in this context. These are mixtures of metal oxides with a basic configuration also consisting of a largely inert porous carrier with a surface on which catalytically active material is applied in dispersed form. Such mixed oxide catalysts are produced by so-called impregnation which may in principle be performed in two ways, set out below.
One option is to apply the catalytically active material on the surface of a porous oxidic carrier by a precipitation reaction. It is also possible that both materials, i.e. both the carrier material and the catalytically active material are precipitated by so-called co-precipitation in which the two materials, i.e. both the carrier material and the catalytically active material are deposited adjacent to one another. Under skillful control of the process the catalytically active material is also deposited on the surface of the inert carrier.
Furthermore it is possible as well to mix a porous carrier material with a solvent and additionally dissolve a metal salt in this solvent. Subsequently, distillation is performed such that the solvent evaporates and the metal salt which is the later active material is deposited on the carrier surface.
In both cases thermal post-treatment follows, in the form of drying and sintering, so that a mixed oxide powder is obtained. This mixed oxide powder already has the heterogeneous structure mentioned above, that means it consists of a carrier on which a layer of catalytically active material is arranged.
Such a material is exemplified in U.S. Pat. No. 3,460,523 A which discloses coatings for cooking appliances that act as oxidation catalysts. The coatings are formed as a porous film, the pores constituting at least 15 vol % of the coating. Furthermore, the coating comprises at least one alkaline silicate binder. The coating is formed by first depositing a catalytically active composition on a carrier which is made of oxides of rare earth elements, for example, e.g. manganese, cobalt, and nickel. This is followed by further thermal processes from which a catalytically active powder is resulting. The so obtained catalytic material is then added to a silicate binder which may additionally contain a filler, for example in the form of silica. Then, a coating is applied to a substrate using the coating solution prepared in this manner.
Furthermore, U.S. Pat. No. 3,993,597 A describes a further development of the aforementioned method. The catalytic composition for coating the surfaces of cooking appliances comprises first a catalytic metal oxide in this case, for example cobalt or manganese oxide, or a rare earth oxide, or mixtures of metal oxides, and furthermore at least 1 wt % of an alkali silicate and a carbonate or hydroxide of at least one catalytic metal oxide. Furthermore, the coating is porous and is deposited from a coating solution consisting of an alkaline silicate binder in which particles of a catalytically active material are dispersed.
German patent application DE 39 42 236 A1 describes a catalytic coating composition which is used for coating cooking, baking, roasting or barbeque equipment or parts thereof. Here, the catalytically active material is, e.g., an oxide of cerium, sodium, potassium, calcium, manganese, nickel, and/or cobalt or mixtures thereof, which may furthermore comprise a silicate. The catalytically active material is highly porous. It is obtained by a precipitation reaction, for example from an aqueous solution. The so obtained material may subsequently be mixed with further film-forming materials, such as an email, and may be applied as a layer, whereby again highly porous coatings are obtained in this way.
Furthermore, German Patent DE 10 2005 009 285 B3 describes a window device and its use for a diagnostic system for combustion processes. The window device in this case comprises at least one window element that comprises an oxidation catalyst material by means of which carbonaceous contaminations of the window can be decomposed. The oxidation catalyst material is selected so that the window contaminations can be decomposed at least within a temperature range from 300° C. to 450° C. The oxidation catalyst material in particular comprises a material selected from group VIIIB, preferably platinum, and is optically transparent.
German patent application DE 10 2007 034 633 A1 describes a coating material with catalytic activity and its use. The coating material is specifically active in view of reducing the combustion temperature of soot and organic substances and is composed of 20 to 90 wt % of compounds of the transition metals or of elements of main groups 3 and 4 and of 10 to 80 wt % of alkali or alkaline earth metal compounds. Additionally, the coating material may be doped with precious metals. The coating material is applied to a substrate and dried. The coating itself is therefore accomplished by a wet-chemical process, for example by conventional wet-chemical coating methods such as flooding, coating with doctor knife, dipping, rolling, printing, or similar methods. The transition elements or elements of main groups 3 and 4 of the periodic table preferably used include zirconium, titanium, aluminum, silicon, cerium, and other metals. The coating solution may be applied directly to a substrate or to a porous carrier.
Finally, German patent application DE 10 2010 050 771 A1 describes an article made of glass or glass ceramic having a high-temperature stable low-energy layer. This layer contains at least one metal oxide of hafnium, yttrium, zirconium, or cerium, or a mixture thereof as a base material, and these metal oxides are present in nanocrystalline form, at least in part. Furthermore, in addition to the first metal oxide the layer contains at least one further metal cation of any of the elements calcium, cerium, yttrium, potassium, lithium, strontium, or gadolinium. The layer produced in this manner has a low-energy surface so that it exhibits both hydrophobic and thermo-catalytic properties, the thermo-catalytic effects occurring already at a temperature of 325° C. and above.
All these materials have in common that they exhibit a number of disadvantages.
For example, with the coatings consisting of a conventional oxidation catalyst deposited on a porous material transparent coatings cannot be obtained or the catalytic activity only starts above a certain temperature, for example above 300° C. Another drawback is that for a number of catalysts precious metals such as platinum are still used.
Therefore, there is a demand for a material which is catalytically effective, especially as an oxidation catalyst, and which can be prepared in the form of a homogeneous material, which is moreover in particular suitable for transparent coatings, and which exhibits a catalytic effect starting already at low temperatures, for example below 300° C.